1. Field of the Invention
The present invention relates to a feedback information transmission/reception method for inter-cell cooperative transmission in a cellular radio communication system and, in particular, to a feedback information transmission/reception method and apparatus in consideration of multiple base stations involved in inter-cell cooperative transmission.
2. Description of the Related Art
The mobile communication system has evolved into a high-speed, high-quality wireless packet data communication system to provide data services and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), both defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in IEEE, have been developed to support the high-speed, high-quality wireless packet data communication services.
Such recent mobile communication systems adopt Adaptive Modulation and Coding (AMC) and channel sensitive techniques to improve transmission efficiency. With AMC, the transmitter can control the data amount according to channel state. That is, when the channel state is bad, the data rate is decreased to math a predetermined error rate, and when the channel state is good, the data transmission rate is increased to match another predetermined error rate. In this way, the mobile communication system can transmit large amount of information efficiently. With the channel sensitive scheduling resource management method, the transmitter can serve the user having superior channel state first selectively among multiple users and thus increase system throughput as compared to the general channel allocation and serving method. For example, the closed-loop precoding, the AMC, and channel sensitive scheduling are the techniques for using the best modulation and coding scheme at the most efficient timing based on the partial channel state information fed back by the receiver.
There has been many researches done to adopt Orthogonal Frequency Division Multiple Access (OFDMA) to next generation communication systems in place of Code Division Multiple Access (CDMA) that has been used in 2nd and 3rd Generation mobile communication systems. The standardization organizations such as 3GPP, 3GPP2, and IEEE are developing standards for enhanced system based on the OFDMA or modified OFDMA. It is known that OFDMA promises to increase system capacity as compared to CDMA. One of the factors affecting the increase of system capacity in an OFDMA system is the use of frequency domain scheduling. As the channel sensitive scheduling technique uses the time-varying channel for capacity gain, it is possible to increase the capacity gain with frequency-varying channel characteristic.
The closed-loop precoding, AMC, and channel-sensitive scheduling are the techniques that are capable of improving the transmission efficiency in the state where the transmitter has acquired information enough on the transmit channel. In FDD (Frequency Division Duplex) mode where the transmitter cannot estimate the state of the transmit channel based on the receive channel, it is designed for the receiver to report the information on the transmit channel to the transmitter. In the mobile communication environment, however, the channel state is time-varying such that efficiencies of the closed-loop precoding, AMC, channel sensitive scheduling techniques are degraded.
The cellular radio communication system is designed such that each base station serves the users within its coverage and hands over the control on the user getting out of the coverage to another base station. In such a cellular structure, the user located at the boundary of a cell experiences interference of the signal transmitted by other base stations such that the channel state is deteriorated. Accordingly, the user close to the base station is served at a high data rate while the user located at the cell boundary is served at a low data rage. In order to solve this problem, it is expected that the 4th generation mobile communication system under discussion adopt the collaborative transmission technique in which multiple base stations transmit signals to the user located at the cell boundary.
Such a cell involved in the collaborative transmission is referred to as collaborative cell. The collaborative transmission can be categorized into one of a low level collaborative transmission in which the collaborative cells perform coordinated scheduling or interference avoidance beamforming to suppress the interference from neighbor cells and a high level collaborative transmission in which the collaborative cells transmit the same signals. The low level collaborative transmission technique makes the scheduling and beamforming decision collaboratively with sharing the real transmission signals. Whereas, the high level collaborative transmission technique allows the collaborative cells to share even the real transmission signals such that the channel state of the user located at the cell boundary is highly improved due to the signal reinforcement rather than interference in spite of the traffic increase in the network.
FIG. 1 is a diagram illustrating a structure of an uplink subframe based on Single-Carrier Frequency Division Multiple Access (SC-FDMA) in an LTE system.
A 10 MHz system bandwidth 103 is composed of total 50 Resource Blocks (hereinafter, referred to as RB). An RB is generated from 12 subcarriers and a basic data transmission scheduling unit. An uplink subframe 101 is composed of 14 SC-FDMA symbol durations 105. Physical Uplink Control Channel (hereinafter, referred to as PUCCH) 106 is transmitted on the RBs at both edges of the system band, and Sounding Reference Signal (hereinafter, referred to as SRS) 109 at the last SC-FDMA symbol 105 across the 10 MHz system band 103. Physical Uplink Shared Channel (hereinafter, referred to as PUSCH) 107 is transmitted in the region with the exception of the PUSCH and SRS regions of the system band, and Reference Signal (hereinafter, referred to as RS) 108 is transmitted on the SC-FDMA symbol in the middle of each slot 103. PUCCH includes Acknowledge/Negative Acknowledge (ACK/NACK) for Hybrid Automatic Repeat Request (HARQ) process, Rank Indicator (RI) for downlink data scheduling, Precoding Matrix Indicator (PMI), and Channel Quality Indicator (CQI); and SRS is the signal for user-specific uplink channel state information acquisition and uplink transmit timing adjustment for the system bandwidth. RS is the signal carrying the channel stat information for use in PUCCH and PUSCH demodulation and decoding.
In order to maintain the single carrier characteristics in uplink transmission, PUCCH and PUSCH are not transmitted in the same subframe. The channel state information can be fed back periodically on PUCCH or non-periodically on PUSCH allocated for feedback in response to the request of the base station.
The precoding matrices defined in multi-antenna based LTE system are shown in tables 1 and 2, and the user terminal reports channel state information including the RI and PMI (codebook index or codebook indicator) corresponding to the RI to the base station through PUCCH or PUSCH.
TABLE 1Codebook Rank index 1 20       1          2        ⁡      [                            1                                      1                      ]        1          2        ⁡      [                            1                          0                                      0                          1                      ]   1       1          2        ⁡      [                            1                                                  -            1                                ]        1    2    ⁡      [                            1                          1                                      1                                      -            1                                ]   2       1          2        ⁡      [                            1                                      j                      ]        1    2    ⁡      [                            1                          1                                      j                                      -            j                                ]   3       1          2        ⁡      [                            1                                                  -            j                                ]  —
TABLE 2CodebookRankIndexun12340u0 = [1 −1 −1 −1]TW0{1}W0{14}/{square root over (2)}W0{124}/{square root over (3)}W0{1234}/21u1 = [1 −j 1 j]TW1{1}W1{12}/{square root over (2)}W1{123}/{square root over (3)}W1{1234}/22u2 = [1 1 −1 1]TW2{1}W2{12}/{square root over (2)}W2{123}/{square root over (3)}W2{3214}/23u3 = [1 j 1 −j]TW3{1}W3{12}/{square root over (2)}W3{123}/{square root over (3)}W3{3214}/24u4 = [1 (−1 − j)/{square root over (2)} −j (1 − j)/{square root over (2)}]TW4{1}W4{14}/{square root over (2)}W4{124}/{square root over (3)}W4{1234}/25u5 = [1 (1 − j)/{square root over (2)} j (−1 − j)/{square root over (2)}]TW5{1}W5{14}/{square root over (2)}W5{124}/{square root over (3)}W5{1234}/26u6 = [1 (1 + j)/{square root over (2)} − j (−1 + j)/{square root over (2)}]TW6{1}W6{13}/{square root over (2)}W6{134}/{square root over (3)}W6{1324}/27u7 = [1 (−1 + j)/{square root over (2)} j (1 + j)/{square root over (2)}]TW7{1}W7{13}/{square root over (2)}W7{134}/{square root over (3)}W7{1324}/28u8 = [1 −1 1 1]TW8{1}W8{12}/{square root over (2)}W8{124}/{square root over (3)}W8{1234}/29u9 = [1 −j −1 −j]TW9{1}W9{14}/{square root over (2)}W9{134}/{square root over (3)}W9{1234}/210u10 = [1 1 1 −1]TW10{1}W10{13}/{square root over (2)}W10{123}/{square root over (3)}W10{1324}/211u11 = [1 j −1 j]TW11{1}W11{13}/{square root over (2)}W11{134}/{square root over (3)}W11{1324}/212u12 = [1 −1 −1 1]TW12{1}W12{12}/{square root over (2)}W12{123}/{square root over (3)}W12{1234}/213u13 = [1 −1 1 −1]TW13{1}W13{13}/{square root over (2)}W13{123}/{square root over (3)}W13{1324}/214u14 = [1 1 −1 −1]TW14{1}W14{13}/{square root over (2)}W14{123}/{square root over (3)}W14{3214}/215u15 = [1 1 1 1]TW15{1}W15{12}/{square root over (2)}W15{123}/{square root over (3)}W15{1234}/2
In table 1, Wn(S) is a matrix composed by taking the columns of the set {s} from Wn=I−2ununH/unHun. Here, I is 4×4 unitary matrix, and un is a vector given in table 2.
In the high level collaborative transmission, the precoding to be used by the collaborative cells is not enough with the conventional codebook designed by taking notice of a single cell. That is, if the codebook designed for a single as shown in tables 1 and 2 is used, it is impossible to expect the coherent combination of the channels of the collaborative cells such that the received signal performance enhancement is limited. Meanwhile, the received signal gain may vary depending on how to use the codebook for collaborative cells. Accordingly, there has been various codebook designs and utilization methods proposed to improve the received signal gain with inter-cell cooperative transmission. Also, there is a need of the method for transmitting feedback information related to the inter-cell cooperative transmission efficiently.